Moves the Lean project to Lean 4.35.0 / Mathlib v4.35.0 to match the runner,
caches the Mathlib project on the runner host, and builds and keeps the Docker
image only after the Python and Lean jobs pass.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FYqwNMyPUHeTbFWg83GQq6
